Using populations of beads for the fabrication of arrays on surfaces

ABSTRACT

The present invention provides a method of creating an array of features. The method can include steps of (a) providing a plurality of beads, wherein each bead in the plurality of beads includes probe content; (b) contacting the plurality of beads with a surface to produce a layer of beads on the surface; and (c) transferring the probe content from the beads to the surface to create an array of spatially discrete features on the surface, wherein each spatially discrete feature includes probe content from a bead in the plurality of beads.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/497,397, filed Jul. 2, 2009 which claims the benefit of U.S.Provisional Patent Application No. 61/077,844 filed Jul. 2, 2008 andU.S. Provisional Patent Application No. 61/149,616 filed Feb. 3, 2009,the disclosures of which are incorporated herein by reference in theirentireties.

STATEMENT REGARDING SEQUENCE LISTING

The Sequence Listing associated with this application is provided intext format in lieu of a paper copy, and is hereby incorporated byreference into the specification. The name of the text file containingthe Sequence Listing is 480273_(—)402a_SEQUENCE_LISTING.txt. The textfile is 3 KB, was created on Sep. 29, 2009, and was submittedelectronically via EFS-Web on Sep. 29, 2009, concurrent with the filingof a Response to Missing Parts in U.S. patent application Ser. No.12/497,397.

FIELD OF THE INVENTION

The present invention relates to methods of fabricating analyticalarrays. More specifically it relates to methods of using beads todeposit analytes onto a surface to form analyte features that areseparated from each other at a desired spacing.

BACKGROUND TO THE INVENTION

The draft sequence of the human genome was published in 2001 by theHuman Genome Consortium (Nature 409; issue 6822) and Celera genomics(Science, 291; 1304-1351), thus marking an important advancement to thegenetics chapter for society. Capitalizing on this investment andrealizing the potential of the Human Genome Project provides a betterunderstanding of genetic variation and its effect in disease.

It has been estimated that any two copies of the human genome differfrom one another by as little as 0.1%, in other words a total of threemillion variants, or one variant every 1000 bases, over a total of threebillion that make up the human genome. Since such variation affectsdisease susceptibility and responses to drugs, it is advantageous toidentify the genetic factors which contribute to biological variation.DNA sequencing is a fundamental tool enabling the screening of genes forsuch genetic mutations associated with disease. High throughput, highaccuracy sequencing methods are therefore beneficial for screening thecomplete genome sequence of an animal in order to identify uniquenucleic acid sequences which may indicate the presence of physiologicalor pathological conditions.

DNA sequencing of large and complex genomes is currently limited bycost. In order to accurately sequence a human genome to a depth of 15×coverage requires the generation of at least 45 billion bases ofsequence. Even for highly parallel sequencing technologies with readlengths of hundreds of base pairs, many hundreds of millions ofsequencing reads are typically obtained in parallel. These reads may berecorded on microscopy based platforms and may therefore involve theconsecutive capturing of many thousands of images on an imaging devicesuch a CCD camera with a finite number of pixels. In order to maximizethe rate of output of sequencing information, efforts have been made toincrease the ratio of bases sequenced per image (i.e., the ratio ofbases/pixels). In general, array techniques that rely on the randomdistribution of features can suffer from a low ratio of bases/pixels,due to a high number of dark pixels with no features (for example, ifthe density of features is too diffuse), or a high number of pixels thatcarry multiple overlapping features of different sequence (if thedensity of features is too concentrated) or both (due to the randomnature of feature placement). A more efficient use of the imaging pixelscan be made if the features on the surface are tightly packed,non-overlapping and of similar size and intensity to each other.

The present invention provides methods of fabricating arrays of featuresthat avoid low ratios of bases/pixels associated with many arrayfabrication methods while exploiting advantages of random featurefabrication. Thus, the presently disclosed invention embodimentsprovide, for example, ease of array fabrication, low cost of arrayfabrication, an increase in the amount of data generated using any of avariety of high throughput imaging platforms, and other relatedadvantages.

SUMMARY OF THE INVENTION

The present invention provides according to certain embodiments a methodof fabrication of an array of nucleic acid fragments by copying thetemplates attached to an array of beads. The beads are attached to asurface via hybridization to a primer on the surface, therebyspecifically selecting the beads with the templates thereon. Theinvention also provides in certain embodiments methods for thefabrication of arrays by using beads to control the density of featureson the array.

One embodiment of the invention involves a method for fabricating anarray of nucleic acids on a surface. The method can include: (a)providing a surface having one or more primer oligonucleotides attachedto the surface; (b) providing a pool of beads, wherein beads in the poolhave a plurality of templates attached thereto, the plurality comprisingmultiple copies of a single nucleic acid template sequence; (c) arrayingthe beads onto the surface by hybridizing the templates to the primeroligonucleotides; and (d) extending the primers to produce copies of thetemplates attached to the surface. In particular embodiments, theprimers are extended under conditions in which no more than one copy ofeach template molecule is produced. For example, single cycle extensionconditions, in which intervening denaturation steps are not applied, canbe used. Alternatively, the primers can be extended under conditions inwhich a particular template molecule is copied several times. Forexample, multiple cycles of extension and denaturation can be carriedout such that a given template can be repeatedly primed and copied.Optionally, the beads can be removed from contact with the surface afterthe primers have been extended to produce the one or more copies of thetemplates attached to the surface.

Accordingly, in certain related embodiments the step (b) of providing apool of beads comprises forming the pool of beads by amplifying thenucleic acid template sequence on the beads, thereby producing theplurality of templates; in certain further embodiments the amplifying isperformed in an emulsion and in certain other further embodiments theamplifying is performed by bridge amplification using two or more primeroligonucleotides immobilized on the beads. In certain furtherembodiments a subset of at least 90% of the beads on the surface carryone or more copies of the single nucleic acid template sequence, and incertain other further embodiments a subset of at least 99% of the beadson the surface carry one or more copies of the single nucleic acidtemplate sequence. In certain further embodiments one or more of theprimer oligonucleotides is cleaved after the amplification to leavesingle stranded templates on the beads.

In certain other embodiments of the above described methods, a subset ofat most 10% of the beads in the pool comprise the copies of the singlenucleic acid template sequence, and in certain other embodiments asubset of at most 1% of the beads in the pool comprise the copies of thesingle nucleic acid template sequence. In another embodiment the copiesof the templates attached to the surface that are produced in step (d)comprise a plurality of nucleic acid features on the surface, whereineach of the features covers less area of the surface than the areacovered by each of the beads. In another embodiment the method furthercomprises removing the beads from the surface after the extending of theprimers.

In certain other embodiments the templates amplified on the beadsoriginate from a pool of chemically synthesized oligonucleotides. Inanother embodiment the method further comprises an additional step ofsequencing the copies of the templates attached to the surface that areproduced in step (d). In another embodiment the primer oligonucleotidesare attached to spatially random locations on the surface. In anotherembodiment the primer oligonucleotides are attached to spatially orderedlocations on the surface. In another embodiment the primeroligonucleotides are attached to the surface in a number per unit areathat exceeds the number of the beads arrayed within the unit area of thesurface. In another embodiment in step (d) the plurality of templatesattached to the beads are each copied two or more times.

Also provided is a device including an array of beads on a surfacewherein the beads are immobilized via hybridization to one or moreoligonucleotide primers on the surface.

The invention further provides a composition including (a) an array ofbeads on a surface, wherein the beads are attached to template nucleicacids, wherein the surface is attached to oligonucleotide primers andwherein the beads are immobilized on the surface via hybridization ofthe template nucleic acids to one or more of the oligonucleotideprimers, and (b) a nucleic acid polymerase that may be bound to hybridsformed between the template nucleic acids and the one or moreoligonucleotide primers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an embodiment of the invention where thetemplates are amplified onto beads.

FIG. 2 shows a schematic of an embodiment of the invention where thebeads are arrayed onto the surface via hybridization, and the templatescopied.

FIG. 3 shows a schematic of an exemplary method whereby templates onbeads can be copied onto a surface. PCR products in solution wereprepared using one biotinylated primer per reaction. The amplicons werecaptured onto one micron streptavidin beads and denatured to make thenucleic acids single stranded. The beads were then flushed into a flowcell and allowed to hybridize to the primers attached to the flow cellsurface.

FIG. 4 shows nucleic acid sequences for two template molecules used inan exemplary PCR reaction.

FIGS. 5A through 5F show white light optical images of surfaces aftercontact with various beads and washing to remove non-hybridized beads(panel A: beads prepared with CT417 (SEQ ID NO:5) template; panel B:beads prepared with CT418 (SEQ ID NO:6) template; panel C: a 1 to 1mixture of beads prepared with CT417 template and beads prepared withCT418 template; panel D: a 1:4 mixture of beads prepared with CT417template and beads prepared with CT418 template; panel E: beads preparedwith a mixture of CT417 and CT418 templates; and panel F: blank beadswith no template attached). As shown by the images beads carryingtemplate sequences hybridize to the immobilized primers on the surface,whereas beads with no templates do not.

FIGS. 6A through 6G show first sequencing cycle images of beads markedin different shades of grey to represent different sequences (panel A:beads prepared with CT417 template; panel B: beads prepared with CT418template; panels C and D: a 1 to 1 mixture of beads prepared with CT417template and beads prepared with CT418 template; panels E and F: a 1:4mixture of beads prepared with CT417 template and beads prepared withCT418 template; and panel G: beads prepared with a mixture of CT417 andCT418 templates). Beads with single templates show spots of a singlegrey intensity with no overlap between adjacent spots. Beads withdifferent templates are shown as different intensity features. The ratioof beads in solution is reflected by the number of spots of the twointensities on the surface. Beads carrying mixed templates still showdiscreet spots, but at a single greyscale in every spot (viewed as asingle color in the original image).

FIG. 7 shows a zoomed in version of a high density array of featuresfabricated using a 1:1 mixture of two different bead types, each beadtype having a different attached template sequence. Both bead types hadan average diameter of 1 micron. The image was obtained followingreplication of bead-bound templates to the array surface and removal ofthe beads. The features on the surface produce a single discrete greycolor intensity, demonstrating that the templates on the beads can becopied onto the surface without overlap of adjacent sequences.Furthermore each spot is a similar size and intensity to other spotsderiving from the same template.

FIG. 8 shows a method of preparing features on a surface by physicallytransferring nucleic acid content from a layer of beads to the surface.

FIG. 9 shows a method of preparing features on a surface by physicallytransferring nucleic acid content from a layer of beads to the surface,wherein a single strand from each of several nucleic acid hybrids onbeads is transferred to the surface.

FIG. 10 shows a method of preparing features on a surface by copyingnucleic acids from a layer of beads using extension of primers on thesurface.

FIG. 11 shows a method of preparing features on a surface by digestingmaterial not in contact with a bead in a layer of beads.

All publications, patents, patent applications, or other documents citedin this application are incorporated by reference in their entirety forall purposes to the same extent as if each individual publication,patent, patent application, or other document were individuallyindicated to be incorporated by reference for all purposes.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides, according to certain herein disclosedembodiments, methods for creating an array of features on a surfacebased on content transferred from a plurality of beads to the surface.Content can be transferred in the form of one or more probe moleculesthat is physically transferred from a bead to a feature of the array.Content transfer can also be carried out by replicating a probe moleculefrom a bead to form a copy of the probe at a feature of the array or bysynthesizing a probe at a feature on the array as directed by a catalystor reagent present on a bead. The use of beads as a medium to transfercontent to a surface allows an array of spatially discrete features tobe formed on the surface. More specifically, a layer of beads can becontacted with a surface such that content present on the beads istransferred to the location of the surface that is closest to the bead.In this way the proximity of beads to each other in the layer determinesthe proximity of the features to each other on the surface. For example,transferring content from a tightly packed monolayer of spherical beadswill produce an array of features that has a center-to-center spacingthat is equivalent to the diameter of the beads. Accordingly, propertiesof the bead layer such as bead shape, bead size and bead packing densitycan be manipulated to obtain a desired pattern of features on an array.

In particular embodiments, the methods can be used to transfer orreplicate one or more copies of a probe molecule from an individual beadto an individual feature of an array. An exemplary type of probe is anucleic acid which can be copied from beads to form an array of nucleicacid probe features. More specifically, a population of beads, in whicheach bead carries a nucleic acid with a particular sequence, can be usedto create features on an array surface, wherein each feature containsone or more copies of a particular nucleic acid sequence from a singlebead in the population. Typically, each bead will have a single probespecies present in one or more copies. Returning to the example ofnucleic acid probes, each bead can have several copies (i.e., severalnucleic acid molecules) with the same sequence (i.e., same species).Alternatively, a single copy of a nucleic acid probe can be transferredor replicated from a bead to a feature of an array. In addition to beinguseful as probes, the nucleic acids that are attached at features of thearray can be used as primers or templates in any of a variety ofarray-based nucleic acid analyses.

It will be understood that a copy of a template nucleic acid made usinga method set forth herein can have a sequence that is identical to thesequence of the template nucleic acid, the copy can have a sequence thatis complementary to the sequence of the template nucleic acid, or both.Accordingly, the copy can be single stranded or double stranded.Similarly, the copy can be derived from a template that is doublestranded or single stranded.

In particular embodiments, the present invention relates to a method offabrication of an array of nucleic acid fragments by copying templatesattached to a plurality of beads. The beads can be attached to thesurface via hybridization to a primer on the surface, therebyspecifically selecting the beads with the templates thereon.

In particular embodiments, the beads may be prepared such that each beadcarries one or more nucleic acid species, for example each bead maycarry two amplification primers each having a different sequence. Thenucleic acids may carry a reactive moiety which allows attachment ofeach nucleic acid to a surface. The beads may be spread out on thesurface, and the moiety on each nucleic acid allowed to react to form acovalent bond with the surface. The beads can then be removed, forexample by cleaving a linker that attaches each nucleic acid to thebead. In this way an array of nucleic acid features is produced on thesurface where the density of features on the array is correlated withthe size and/or spacing of the beads. The nucleic acids that aretransferred from the beads to the surface can be single stranded (asshown for example in FIG. 8) or they can be double stranded.

In embodiments wherein beads are attached to double stranded templates,one of the two strands in each template can be transferred to a surface.For example as shown in FIG. 9, beads having double stranded templatesin which one strand comprises a reactive moiety can be arrayed on asolid support and the hybridized material reacted with the surface. Thearray is therefore fabricated by the transfer of one strand of thehybridized material from the beads to the surface, and the density offeatures is correlated with the size and/or spacing of the beads. Thebeads can be removed from the surface by denaturation, and can bere-used if desired by ‘inking’ with more hybridized material.

An array may also be fabricated using hybridization of one or morenucleic acids on beads to one or more primers on a surface and extensionof the one or more primers using the one or more nucleic acids astemplates. An exemplary diagram is shown in FIG. 11. For example, thebeads may carry a nucleic acid having a sequence of e.g., 20nucleotides. The surface may carry a complementary nucleic acid having asequence of e.g., 8 or 12 nucleotides. The nucleic acids on the beadscan therefore hybridize to the nucleic acids on the surface at lowtemperatures. The 20-mer nucleic acid sequences can be extended using apolymerase and nucleotide triphosphates such that the full lengthprimers on the surface are only prepared where the beads are in contactwith the support. Alternatively, the primers on the surface can beextended by ligation using an oligonucleotide complementary to thesequence on the beads. The array will have areas of full lengthsequences and shorter sequences and can be used for subsequent assayswhere features are only produced in regions of full length sequences.For example in the preparation of nucleic acid clusters byamplification, clusters can be isolated from each other because theshort sequences are too short to enable bridge amplification, and hencethe clusters can not cross the gaps between the islands of longsequences. In such examples, the template on the bead may be asynthesized oligonucleotide sequence. The template may, for example,comprise an artificial oligonucleotide sequence of between 20-50 basepairs.

Arrays may also be fabricated by using beads to either protect oractivate certain regions of a surface. For example, content attached toone or more beads can bind to content present on a surface toselectively protect the content on the beads that is in contact withcontent on the surface, the content on the surface that is in contactwith the content on the beads, or both, from degradation ormodification. Alternatively or additionally, content attached to one ormore beads can react with content present on a surface to selectivelymodify the content on the beads that is in contact with content on thesurface, the content on the surface that is in contact with the contenton the beads, or both. Taking as an example beads having one or morenucleic acid templates that are complementary to nucleic acid primers ona surface, the beads can be located on the surface and the surfacetreated with exonuclease under conditions wherein the beads offerprotection from exonuclease treatment such that the primers can beselectively digested on the regions of surface between the beads.

As a further example, beads may carry an enzyme or other moiety thatreacts with primers on a surface that are in contact with the beads.More specifically, the beads may carry a phosphatase to remove aphosphate group from primers at certain locations of the surface thatare on contact with the beads, thereby forming features havingdephosphorylated primers. Alternatively the beads may carry anexonuclease or endonuclease to digest the surface bound primers thatcome into contact with the beads such that regions that do not come intocontact with the beads form features having full length (i.e.,non-digested) primers. The dephosphorylated primers, or full lengthprimers at the respective features can then be used to produce nucleicacid clusters, whereas the phosphorylated, or shortened primers are notamenable to further extension. Alternatively the beads may carry achemical reagent such as a phosphine which induces a chemicalmodification to the primers on the surface. In the cases of beadscarrying reagents to alter the surface bound primers, the beads may alsocarry nucleic acid sequences which hybridize to the surface, thuscausing the beads to be immobilized onto the surface rather than simplydiffusing in solution.

As used herein, the term “array” means a population of different probemolecules that are attached to a surface such that the different probemolecules can be differentiated from each other according to relativelocation. An individual feature of an array can include a single copy ofa probe molecule or multiple copies of the probe molecule can be presentas a population of probes at an individual feature of the array. Thepopulation of probes at each feature typically is homogenous, having asingle species of probe. Thus, multiple copies of a single nucleic acidsequence can be present at a feature, for example, on multiple nucleicacid molecules having the same sequence. However, in some embodiments aheterogeneous population of probes can be present at a feature. Thus, afeature may but need not include only a single probe species and caninstead contain a plurality of different probe species such as a mixtureof nucleic acids having different sequences.

Neighboring features of an array can be discrete one from the other inthat they do not overlap. Accordingly, the features can be adjacent toeach other or separated by a gap. In embodiments where features arespaced apart, neighboring sites can be separated, for example, by adistance of less than 100 μm, 50 μm, 10 μm, 5 μm, 1 μm, 0.5 μm or less.The layout of features on an array can also be understood in terms ofcenter-to-center distances between neighboring features. An array usefulin the invention can have neighboring features with center-to-centerspacing of less than about 100 μm, 50 μm, 10 μm, 5 μm, 1 μm, 0.5 μm orless. Furthermore, it will be understood that the distance valuesdescribed above and elsewhere herein can represent an average distancebetween neighboring features of an array. As such, not all neighboringfeatures need to fall in the specified range unless specificallyindicated to the contrary, for example, by a specific statement that thedistance constitutes a threshold distance between all neighboringfeatures of an array.

The methods set forth herein can be used to make arrays having featuresat any of a variety of densities. Very high density arrays are useful inthe invention including, for example, those having from about 10,000,000features/cm² to about 2,000,000,000 features/cm² or from about100,000,000 features/cm² to about 1,000,000,000 features/cm². Highdensity arrays include, for example, those in the range from about100,000 features/cm² to about 10,000,000 features/cm² or about 1,000,000features/cm² to about 5,000,000 features/cm². Moderate density arraysrange from about 10,000 features/cm² to about 100,000 features/cm², orfrom about 20,000 features/cm² to about 50,000 features/cm². Low densityarrays are generally less than 10,000 features/cm² with from about 1,000features/cm² to about 5,000 features/cm² being useful in particularembodiments. Very low density arrays having less than 1,000features/cm², from about 10 features/cm² to about 1000 features/cm², orfrom about 100 features/cm² to about 500 features/cm² are also useful insome applications.

As used herein, the term “surface” means a part of a support structurethat is accessible to contact with reagents, beads or analytes. Thesurface can be substantially flat or planar. Alternatively, the surfacecan be rounded or contoured. Exemplary contours that can be included ona surface are wells, depressions, pillars, ridges, channels or the like.Exemplary materials that can be used as a support structure include, butare not limited to, glass such as modified or functionalized glass;plastic such as acrylic, polystyrene or a copolymer of styrene andanother material, polypropylene, polyethylene, polybutylene,polyurethane or Teflon™; polysaccharides or cross-linked polysaccharidessuch as agarose or Sepharose; nylon; nitrocellulose; resin; silica orsilica-based materials including silicon and modified silicon;carbon-fibre; metal; inorganic glass; optical fibre bundle, or a varietyof other polymers. A single material or mixture of several differentmaterials can form a surface useful in the invention.

A surface used in the invention may be contained in a flow chamberallowing convenient movement of liquids across the surface to enable thetransfer of reagents. Exemplary flow cells that can be used aredescribed in WO 2007/123744, which entered the U.S. national phase asU.S. patent application Ser. No. 12/295,337, each of which isincorporated herein by reference.

As set forth herein, transfer of content from beads to a surface canoccur due to interaction of the surface with the content on the beads.Thus, the surface can include moieties or groups that interact with thecontent on beads that is to be transferred. The interactions can becovalent or non-covalent in nature. For example, the surface can includereactive groups that will form a covalent bond with reactive groups onthe beads. Some useful reactive groups and reactions for formingcovalent bonds are set forth in further detail below in regard toattaching nucleic acids to surfaces or beads. In embodiments whereintransfer of content from beads to a surface occurs via non-covalentinteractions, the surface can include groups or moieties such as nucleicacids having sequences that are complementary to sequences of nucleicacids on the beads, ligands having affinity for receptors on the beadsor receptors having affinity for ligands on the beads. Exemplaryreceptor-ligand pairs that can be used include without limitationantigen and immunoglobulin or active fragments thereof, such as FAbs;immunoglobulin and immunoglobulin (or active fragments, respectively);avidin and biotin, or analogues thereof having specificity for avidinsuch as imino-biotin; streptavidin and biotin, or analogues thereofhaving specificity for streptavidin such as imino-biotin; andcarbohydrates and lectins.

Once content has been transferred from beads to a surface by covalent ornon-covalent attachment of the content bearing material to the surface,the material can be detached from the beads. Detachment can be carriedout by a method that is appropriate for the type of bead attachment. Forexample a non-covalent attachment to beads can be disrupted usingdenaturation methods or conditions that prevent the non-covalentinteractions. In embodiments that utilize covalent attachment ofmaterial to beads the material can be cleaved from the beads usingchemical cleavage, enzymatic cleavage, photolysis or the like.

Nucleic acids can be immobilized to a bead or other surface by singlepoint covalent attachment to the surface at or near the 5′ end of thenucleic acid. In embodiments where the nucleic acid serves as a primer,attachment is configured to leave the template-specific portion of theprimer free to anneal to its cognate template and the 3′ hydroxyl groupfree for primer extension. Any suitable covalent attachment means knownin the art may be used for this purpose. The chosen attachment chemistrywill depend on the nature of the solid support, and any derivatizationor functionalization applied to it. The primer itself may include amoiety, which may be a non-nucleotide chemical modification, tofacilitate attachment. In a particular embodiment, the primer mayinclude a sulphur-containing nucleophile, such as phosphorothioate orthiophosphate, for example, located at the 5′ end. In the case ofsolid-supported polyacrylamide hydrogels (as described below), thisnucleophile will bind to a bromoacetamide group present in the hydrogel.A more particular means of attaching primers and templates to a solidsupport is via 5′ phosphorothioate attachment to a hydrogel comprised ofpolymerized acrylamide and N-(5-bromoacetamidylpentyl) acrylamide(BRAPA), as described fully in U.S. application Ser. No. 10/585,373,deriving from WO05065814, the contents of which are incorporated hereinby reference in their entirety. Attachment can also occur via ligandreceptor interactions such as those set forth elsewhere herein. Hydrogelsurfaces are especially advantageous for the transfer of material fromthe bead to the surface. A porous, flexible hydrogel surface, where thebeads can embed into the surface, can allow transfer of higher amountsof material from the bead to the hydrogel due to the increasedsurface-surface contact area as compared to the surface area availablefor a rigid planar surface.

Certain embodiments of the invention may make use of solid supports orbeads comprised of an inert substrate or matrix (e.g., glass slides,polymer beads, etc.) which has been “functionalized”, for example byapplication of a layer or coating of an intermediate material comprisingreactive groups which permit covalent attachment to biomolecules, suchas polynucleotides. Examples of such supports include, but are notlimited to, polyacrylamide hydrogels supported on an inert substratesuch as glass. In such embodiments, the biomolecules (e.g.,polynucleotides) may be directly covalently attached to the intermediatematerial (e.g., the hydrogel), but the intermediate support material mayitself be non-covalently attached to the substrate or matrix (e.g., theglass substrate). It will be understood that a solid support that isuseful in the invention can have several layers and that the layers canhave different compositions.

Any of a variety of methods can be used to prepare a surface for use ina method of the invention. Such methods can be used to deposit groups ormoieties uniformly across all or part of a surface. In some embodiments,groups or moieties can be attached in a particular pattern. However,typically patterned deposition of groups and moieties on a surface to beused for bead-based transfer is not necessary because bead transfer isused to create a pattern of features on the surface.

In particular embodiments groups or moieties are present on a surface ata density that exceeds the density attainable by a monolayer of beadsthat are contacted with the surface. For example, in embodiments wheretemplate-bearing beads are contacted with a surface having attachedprimers, the primers can be attached to the surface at a higher densitythan the density of beads that contact the surface such that each beadis proximal to a plurality of the surface-attached primers. Thus aplurality of templates on each bead can hybridize to a plurality ofprimers on the surface. A plurality of surface-attached primers that isproximal to each individual bead can be, for example, at least 2; 10;100; 1,000; 10,000; 100,000 or more primers depending upon the size ofthe bead and density of primers on the surface. Accordingly, a pluralityof templates that is copied to a feature can be at least 2; 10; 100;1,000; 10,000; 100,000 or more templates. Similar densities can be usedfor other moieties used for transfer of other materials from beads in amethod of the invention according to certain embodiments.

As used herein, the term “bead” means a small body made of a rigid orsemi-rigid material. The body can have a shape characterized, forexample, as a sphere, oval, microsphere, or other recognized particleshape whether having regular or irregular dimensions. Exemplarymaterials that are useful for beads include, without limitation, glasssuch as modified or functionalized glass; plastic such as acrylic,polystyrene or a copolymer of styrene and another material,polypropylene, polyethylene, polybutylene, polyurethane or Teflon™;polysaccharides or cross-linked polysaccharides such as agarose orSepharose; nylon; nitrocellulose; resin; silica or silica-basedmaterials including silicon and modified silicon; carbon-fiber; metal;inorganic glass; optical fiber bundle, or a variety of other polymers.Exemplary beads include controlled pore glass beads, paramagnetic beads,thoria sol, Sepharose beads, nanocrystals and others known in the art asdescribed, for example, in Microsphere Detection Guide from BangsLaboratories, Fishers Ind. Beads can be made of biological ornon-biological materials. Magnetic beads are particularly useful due tothe ease of manipulation of magnetic beads using magnets at varioussteps of the methods described herein.

The invention provides methods that allow for the transfer of materialfrom a bead to a surface. Any of a variety of materials can be presenton a bead and used for transfer including, for example, the groups andmoieties set forth above in regard to surfaces. Similarly the methodsset forth above for preparing surfaces can be used to functionalizebeads for use in the methods of the invention. The use of templatenucleic acids on the beads to hybridize to surface-attached primersallows efficient capture of the beads, such that any beads that lacktemplates can be removed since they will not hybridize. In particularembodiments, the beads are captured to form a single layer, the primershybridized to the templates are extended, and the beads are removedafter the primer extension reaction has occurred. In such embodiments,the extended primers on the surface are derived from the beads, andtherefore the features produced may have minimal overlap with eachother.

Using the bead-based transfer methods set forth herein, each featuregenerated on the surface of an array can be of similar or smaller sizethan the area of the surface occupied by the bead from which the featurewas produced, and all the features will typically be of similar size andintensity to each other. The uniform size, uniform intensity and lack ofoverlap can provide a usable density of features per unit area that ishigher than that obtainable from other random deposition methods sincethese other methods often give rise to a surface where the features maybe overlapping and of different sizes and intensities. Images of tightlypacked non overlapping arrays with features of uniform size andintensity are typically easier to analyze than images where a subset ofthe features overlap with each other.

Accordingly, the methods set forth herein provide several advantages notprovided by other random deposition methods. In this regard it will beunderstood that there are at least two types of order for an arraydescribed herein, the first relating to the spacing and relativelocation of attachment sites and the second relating to identity orpredetermined knowledge of the particular species of molecule thatattaches at a particular site. An array fabricated using the methods setforth herein can be ordered in one respect and random in another. Forexample, in several embodiments set forth herein a substrate iscontacted with a population of nucleic acids under conditions where thenucleic acids attach at sites that are ordered with respect to theirrelative locations but random with respect to knowledge of the sequencefor the nucleic acid species present at any particular site.Accordingly, features of an array can be randomly located such thatnearest neighbor features have random spacing between each other.Alternatively the spacing between features can be ordered, for example,forming a regular pattern. In another respect, features of an array canbe random with respect to the identity or predetermined knowledge of thespecies of analyte that occupies each feature independent of whetherspacing produces a random pattern or regular pattern.

Beads exposed to the surface of an array substrate in a method accordingto certain embodiments of the invention may be present as a mixture ofbeads with and without nucleic acid molecules attached. The nucleic acidmolecules may include attached primers and/or templates. Depending uponthe methods used to prepare the beads and amplify the templates theremay be beads with no nucleic acid molecules at all, beads with primersand no template, beads with both primers and nucleic acid templates, oreven beads that carry only templates due to the conversion of all theprimers to templates in an amplification reaction. The surface to whichthe beads are to be contacted for transfer of template sequences mayhave attached primers that are complementary to the ends of thetemplates such that the beads are captured onto the surface viahybridization to the templates. Removal of the beads without templateswhich do not hybridize allows the ratio of beads with templates to beadswithout templates to be different from the ratio of the beads in themixture that was initially applied to the surface. The beads in solutionwithout templates can therefore be removed simply by removing thesolution. Optionally, the array surface can subsequently be washed underconditions where nucleic acid hybrids between bead-bound templates andprimers on the array surface are maintained whereas beads that do notinteract with the surface or those having only non-specific interactionwith the surface are removed.

In particular embodiments, beads having templates that are complementaryto primers on a surface can be selectively immobilized to the surfacewhile beads without templates can be removed due to the selectivehybridization conditions. Beads having attached templates will beretained on the surface by hybridization of the templates to theprimers. Beads without attached templates will not be retained and canbe washed away such that the majority of the beads on the surface have atemplate attached. For example, greater than 90% of the beadsimmobilized on the surface may carry a template, or even greater than99% of the immobilized beads may carry a template.

In particular embodiments, beads are captured to a surface as a singlelayer. An advantage of capturing beads as a single layer is that thenucleic acid template features on the surface are at a uniform focaldepth, and at a single ‘monolayer’ depth of beads on the surface. Theuse of the templates on the beads to affect capture by hybridizationallows the beads to be captured to form a tightly packed array of beads,and the beads which are physically held above the surface are notcaptured, thus forming a bead monolayer on the surface. The phrase“captured to a single layer” therefore means that the beads are held onthe surface at a depth approximating only a single bead. In other words,beads captured to a single layer will be held on the surface in a layerhaving a depth that is no greater than the diameter of the beads.Typically, when beads are captured to a single layer the number of beadsimmobilized on the surface will be less than or equal to the number ofbeads that would theoretically fit into that area of surface.

The features on a surface may be fabricated using the methods set forthherein such that the features do not overlap with each other. Contenttransferred from a bead to a surface will typically occupy a discretelocation of the surface that is proximal to the bead. Thus the featuresthat come from one bead typically will not merge into features comingfrom a different bead. Accordingly, the images seen on the surface willshow discrete features, the size and shape of which will mirror the sizeand shape of the beads in a monolayer when viewed from above. Forexample, if the beads used to transfer content to a surface arespherical, then the features on the surface will appear as circles. Incontrast, features that are merged together may appear as ovals orfigures of eight. Following bead-based transfer there may be one or moredark pixels between one feature and the next. The images may beconstructed such that each feature occupies a single pixel, or a squareof pixels (e.g., 4, 9, 16, etc.), with no dark pixels between features.One advantage of non-overlapping arrays over overlapping arrays is theability to reduce the number of pixels between images. In particularlyuseful embodiments, the number of features will mirror the number ofpixels on the detector, and hence the imaging is 100% efficient in termsof features per pixels. Non-overlapping features therefore include thosein which the template sequence, transferred to a surface from one beaddoes not become intermingled with the template sequence, from adifferent bead.

As set forth above, the features on the surface of an array canoriginate from a bead in solution. The features may be copied ortransferred from the beads to the surface. As the copying ortransferring utilizes molecular contact between the beads and thesurface, the area of the surface to which the bead is able to interactwill typically be similar or less than the size of the beads. Thus eachfeature on the surface can be of similar or smaller size compared to thecross-sectional area of the beads. The beads may be spherical in size,in which case this can be described in terms of diameter, or may beother shapes, in which case this can be described in terms of area. Thearea of the surface covered by the features will typically be no greaterthan the area of the surface covered by the bead.

In particular embodiments, features on a surface which originate frombeads will be of similar size to each other. Variation between the sizesof the features will typically reflect the variation in the sizes of thebeads. Accordingly, a spherical bead pool with a diameter of 1 micronplus/minus 25% typically gives circular features on a surface that areon average 1 micron diameter but varying in diameter by 25%. Thevariation in area between the different features may, for example, beless than 25%, or less than 10%, or less than 5%, or less than 1%. Ifdesired, arrays having a larger variation in feature area can befabricated.

Features on a surface that are transferred from beads may be smallerthan the area of the surface that is (or was) occupied by the beads. Thedensity of beads, and therefore features, may be tightly packed. Thedensity of beads may, for example, be at least 50% of the availablesurface area, or at least 60% of the available surface area, or at least70% of the available surface area, or at least 80% of the availablesurface area or more. The beads may be disposed in a flow chamber in aclosely packed planar array. As used herein, “closely packed” inreference to a pool of spherical beads in contact with a surface, meansthat the number of spherical beads per unit area in a layer of the poolthat is adjacent to the surface is at least eighty percent of the numberof the spherical beads in a hexagonal array of equal area. As usedherein, a “hexagonal” array of spherical beads means an array ofspherical beads in which every bead in the array contacts six otheradjacent beads of the array.

Typically, the average distance between centers of adjacent beads in aclosely packed pool of beads is less than two bead diameters or beadwidths. If desired, closer packing can be used such that the averagedistance between centers of adjacent beads is less than one and a halfbead diameters or bead widths. However, more diffuse packing is alsopossible. For example, the average distance between centers of adjacentbeads in a pool of beads can be less than 3, 4, 5, or 10 bead diametersor bead widths. More diffuse packing can be achieved, for example, byincluding blank beads into a monolayer of content-bearing beads.Locations in the monolayer occupied by blank beads will be incapable oftransferring content. Thus, the locations on the surface that areproximal to the blank beads will be devoid of content-bearing features.For example, in embodiments wherein templates are copied from beads to asurface, addition of blank beads to the layer of template-bearing beadsthat contact the surface will reduce the density of features copied tothe surface.

In particular embodiments it is desired that upon labelling, differentfeatures on a surface which originate from different beads shouldincorporate a similar number of labels, and thus be of similar intensityto each other when imaged. As set forth above, conditions for transferof template sequences from beads to a surface can be selected to providefeatures having uniform average area. Uniform labelling conditions canbe used such that the number of molecules copied or transferred from thebead to the surface reflects the intensity of the features on thesurface when each of the molecules is labelled. The intensity of thefeatures may therefore fall within a narrow distribution. For example,the variations between the brightest and weakest feature may be lessthan 50%, or less than 25%, or less than 10% or less than 5% or lessthan 1%.

An embodiment of the invention may include a step of attaching nucleicacid templates to a population of beads, for example, prior to using thebeads to fabricate an array. The templates may be present in solutionand captured onto the beads, amplified in the presence of the beads, oramplified on the beads. The amplification on the beads may be done usingan immobilized primer and a primer in solution, or with one or moreimmobilized primers via bridge amplification. Bridge amplification canbe carried out under conditions wherein primers that participate inamplification are not provided in solution. The templates may bemodified so that the ends of the templates carry universal sequencessuch that several different templates can be amplified using the sameuniversal primers. Examples of methods of attaching universal ends to acollection of target fragments can be found in US application US2007-0128624, the contents of which are incorporated herein by referencein their entirety.

Templates may be amplified on beads, in which case the method ofamplification dictates the configuration of the primers on the beads. Inorder to use emulsion-based amplification techniques with a singletemplate per emulsion bubble, a single primer is attached to the bead,and a single primer is in solution, thereby amplifying the templatessuch that one end of the duplex is attached to the bead. The hybridizedstrand can be removed by denaturing the duplex, thereby leaving theimmobilized single strand on the bead. The single stranded templates canbe captured onto a surface via primers complementary to the templates.Exemplary emulsion-based amplification techniques that can be used in amethod of the invention are described in US 2005/0042648; US2005/0079510; US 2005/0130173 and WO 05/010145, each of which isincorporated herein by reference.

Alternatively templates that are attached to beads may be amplifiedusing bridge amplification on the beads, in which case all primers usedfor amplification are immobilized on the beads. An exemplary bridgeamplification method is shown in FIG. 1. Typically, bridge amplificationis carried out in conditions wherein there is no nucleic acid materialin solution, thereby avoiding transfer between beads during theamplification cycles and cross-contamination of the beads. The templatescan be prepared to have a moiety which allows direct attachment of thetemplates to the beads, for example a biotin moiety or a chemicalspecies such as a thiophosphate or amino group. Alternatively, as shownin FIG. 1, the templates can be hybridized to primers on the beads andthe primers extended followed by removal of the originally hybridizedtemplates. Bridge amplification gives a double stranded template whereboth ends are immobilized. In order to obtain a single stranded templatesuitable for hybridization, one of the strands can be cleaved from thesurface. The cleavage may be a chemical treatment of a suitable modifiedstrand, for example periodate treatment of a primer carrying a diolmodification, or reduction of a primer carrying a disulfide, or may bean enzymatic treatment, for example using a restriction endonuclease, oran enzyme treatment to produce an abasic nucleotide site followed bystrand scission. Such treatments may be carried out on primers with auracil modification using uracil DNA glycosylase and an endonuclease, oron primers with an 8-oxo guanine modification using FPG(formamidopyrimidine-DNA glycosylase). In cases where specificnucleotides are used in the cleavage, the nucleotides may be introducedto the surface primers as part of the extension reaction opposite thetemplates on the bead. Thus the extension reaction may be carried outusing, for example, dUTP or 8-oxodGTP to introduce sites forlinearization which are specific to the extended strands rather thanuniversal across all the primer sequences attached to the support.Methods of bridge amplification that include steps to remove one strandof a duplex after bridge amplification are described in co-pendingapplication published in the PCT as WO/2007/010251, the US nationalphase application of which published as US 2009/0118128, the contents ofwhich are incorporated herein in their entirety.

In methods where templates are amplified on beads, it is advantageous toreduce the number of beads which carry multiple templates of differentsequence, thereby enriching for beads having only a single templatemolecule attached. This may be achieved by using an excess of beads totemplate molecules during an amplification or attachment step, therebyensuring the majority of the beads have no templates molecules at all.If the beads are present at a 9/1 excess over template nucleic acidmolecules, then the probability is that 90% of the beads will carry notemplate, and only 1% of the beads will carry two templates. If thebeads are present at a 99/1 excess, then 99% of the beads will probablycarry no template, and only 0.01% of the beads will probably carry twotemplates.

The terms ‘target nucleic acid sequence’, ‘target nucleic acidmolecule’, ‘target nucleic acid’ and ‘target nucleic acid fragment’ maybe used interchangeably to refer to nucleic acid molecules or sequencesthat it is desired to sequence or otherwise analyze. A target nucleicacid can be attached to an array using the bead transfer methods setforth herein. Alternatively, a probe nucleic acid can be attached to anarray by bead transfer methods and the array used subsequently to detecta target in a sample that interacts with the probe. In this regard, itwill be understood that the terms “target” and “probe” can be usedinterchangeably with regard to nucleic acid detection methods, unlessindicated to the contrary. The nucleic acid target may be essentiallyany nucleic acid of known or unknown sequence. It may be, for example, afragment of genomic DNA or cDNA. Sequencing may result in determinationof the sequence of the whole, or a part of the target molecule. Thetargets can be derived from a primary nucleic acid sample. The primarynucleic acid from which targets are derived can first be randomlyfragmented. The targets can be processed into templates suitable foramplification by the placement of universal amplification sequences atthe ends of each target fragment. The targets can also be obtained froma primary RNA sample by reverse transcription into cDNA. Templates mayinclude any nucleic acid sequence capable of being copied by extending aprimer. Templates may derive from biological sources, or be synthesizedas oligonucleotides. Templates will generally have a region of knownsequence such that a primer can be hybridized at a specific location onthe template strand.

As used herein, the term ‘nucleic acid’ refers to deoxyribonucleic acid(DNA), ribonucleic acid (RNA), analogues of either DNA or RNA made fromnucleotide analogs and the like. The term is applicable to singlestranded (such as sense or antisense) and double strandedpolynucleotides. The term as used herein also encompasses cDNA, that iscomplementary or copy DNA produced from an RNA template, for example bythe action of reverse transcriptase.

Primary target nucleic acid molecules that are attached to a bead fortransfer to a surface may have originated in double-stranded DNA (dsDNA)form (e.g., genomic DNA fragments, PCR and amplification products andthe like) or may have originated in single-stranded form, as DNA or RNA,and been converted to dsDNA form. By way of example, mRNA molecules maybe copied into double-stranded cDNAs suitable for use in the method ofcertain embodiments of the invention using standard techniques wellknown in the art. The precise sequence of the primary nucleic acidsmolecules may be known or unknown.

In a particular embodiment, the primary target and/or template nucleicacid molecules are DNA molecules. More particularly, the primary targetand/or template nucleic acid molecules represent the entire geneticcomplement of an organism, and are genomic DNA molecules which includeboth intron and exon sequences (coding sequence), as well as non-codingregulatory sequences such as promoter and enhancer sequences. Althoughit is also possible that particular sub-sets of nucleic acid sequencesor genomic DNA can be used, such as, for example, particularchromosomes. Yet more particularly, the sequence of the primary nucleicacid molecules need not be known. Still yet more particularly, theprimary nucleic acid molecules are human genomic DNA molecules. The DNAtarget molecules may be treated chemically or enzymatically, eitherprior to or subsequent to any random fragmentation processes, and priorto or subsequent to the ligation of adaptor sequences to target nucleicacid molecules. In the methods described herein, the nucleic acidsamples may be fragmented prior to hybridization on beads, or may beused without fragmentation, and may be used with and without adapters.The samples may be subjected to amplification prior to use, for example,a whole sample amplification technique such as random primer extension.

Random fragmentation refers to reducing the length of a nucleic acidmolecule in a non-ordered fashion, for example, by enzymatic, chemicalor mechanical means. Such fragmentation methods are known in the art andcan utilize standard methods (Sambrook and Russell, Molecular Cloning, ALaboratory Manual, third edition). Moreover, random fragmentation isdesigned to produce fragments irrespective of the sequence identity orposition of nucleotides comprising and/or surrounding the break. Moreparticularly, the random fragmentation is by mechanical means such asnebulization or sonication to produce fragments of about 50 base pairsin length to about 1500 base pairs in length, still more particularly50-700 base pairs in length, yet more particularly 50-400 base pairs inlength.

A nucleic acid used in a method of the invention, such as a templatethat is to be transferred from a bead to a surface, can include auniversal sequence. The term “universal sequence” means a region ofsequence that is common to two or more nucleic acid molecules where themolecules also have regions of different sequence. A universal sequencepresent in different members of a collection of molecules can allow thereplication or amplification of multiple different sequences using asingle universal primer complementary to the universal sequence. Thus auniversal primer includes a sequence that can hybridize specifically toa universal sequence. The nucleic acid target molecules may be modifiedto attach universal adapters to one or both ends of the different targetsequences, the adapters providing sites for hybridization of universalprimers.

At least a portion of a universal primer will be complementary to theuniversal sequence to which it hybridizes. Thus universal adaptersequences attached to the ends of double stranded target fragments maycarry a region of non complementary sequence. This non-complementarysequence can be used to hybridize a priming sequence, which can be usedto copy one of the target strands. The primer may carry a tailedoverhang, such that the 3′ end of the adapter may also be extendedcomplementary to the tailed primer. This tailed nature of the primer canbe configured to allow selective introduction of a sequence at one endof the template strands, and can make the 3′ and 5′ ends of thetemplates different to each other and non-complementary to each other.The template strands may be amplified in solution prior to amplificationon beads in order to generate a library of templates of differentsequence comprising common 3′ ends and common 5′ ends.

A template library made by methods set forth herein or other methodsknown in the art may be attached to a pool of beads. The templates maybe prepared in solution and attached to the beads, for example if thebeads carry different nucleic acid sequences to selectively capture thetemplates. The nucleic acid sequences may be synthesized on the beads,or may be prepared and attached to the beads post synthetically. Eachbead can carry a single sequence to enable emulsion-based PCRamplification, or each bead can carry two or more sequences to enablebead-based bridge amplification. The nucleic acid sequences may beattached covalently, for example using an amino modified nucleic acidand a carboxylate bead, or non-covalently, for example usingbiotin-modified nucleic acids and streptavidin beads. Bead-based bridgeamplification of specific sequences is described in U.S. Pat. No.6,090,592 and U.S. Pat. No. 5,641,658, the contents of which areincorporated herein by reference in their entirety.

Amplification of immobilized nucleic acids may be carried out by anyknown technique. Known techniques include thermocycling, where thetemperature of a solution is altered to cycle between primerhybridization, extension and denaturation or isothermal amplificationwherein the temperature is unchanged through the amplification reaction.Isothermal amplification techniques include cycles of buffer exchangewith a denaturant such as formamide or hydroxide, and an extension mixcomprising a polymerase and nucleoside triphosphates, as described in US2008/0009420, the contents of which are incorporated herein in theirentirety, or methods where the amplification is performed in a singlesolution which comprises, for example, a helicase or strand-displacingpolymerase.

If desired, templates on beads may be amplified using helicase dependentamplification techniques. In methods of bridge amplification on beads,it may be advantageous to force as many primers as possible on the beadto be extended. As the original template is only present as a singlecopy, this may require a large number of cycles of amplification. Ratherthan performing hundreds of cycles of denaturation, renaturation andextension, the use of a helicase to denature the double strands, and apolymerase in combination in a single reagent composition, means thatthe beads can simply be left for a period of time and the bridgeamplification allowed to proceed at a single temperature, withoutchanging the reagents or having to separate the beads from the solution.Thus the beads can simply be left to extend all the primers on thesurface rather than having to vary the conditions or manipulate thebeads. Therefore certain embodiments of the invention include the use ofhelicase dependent amplification for amplifying single templates onbeads. Helicase amplification methods can similarly be used to amplifynucleic acid sequences on a surface, for example, after beingtransferred from beads. An alternative method to amplify the templateson the beads is Recombinase Polymerase amplification (RPA) whereby arecombinase enzyme is used to anneal a second primer once the firstprimer has been extended. RPA is more fully described in U.S. Pat. No.7,485,428, the contents of which are incorporated herein by reference intheir entirety.

Beads may be prepared such that all the beads in a pool carry primershaving the same sequence(s). If template nucleic acids are modified tocarry universal ends, which may be complementary to the sequences of theprimers attached to the beads, then the sequences on the beads can beused to amplify the templates. The primers may be synthesized on thebeads, or may be attached post-synthetically. For example, primers maycarry a biotin moiety for attachment to a streptavidin bead, or an aminofunctionality for attachment to a carboxylate bead. Any attachmentmethod used to attach primers or other analytes to the beads is withinthe scope of these and related invention embodiments providing theattachment remains stable during the processes of the inventionembodiments. Exemplary attachment methods and groups that can be usedfor modifying beads include, but are not limited to, those exemplifiedpreviously herein with regard to surface functionalization.

Alternatively, in certain embodiments the beads may be constructed suchthat different beads carry different primers. A patterned surfacecreated using such beads will give patches of primers such that adjacentpatches comprise primers of different sequence. Upon amplification usingthe primers on the support, it is possible that the templates beingamplified can cross between adjacent patches, or features, if thepatches are close together and the templates carry universal endscomplementary to the patches. It is possible to increase the occupancyof the patches whilst reducing the number of patches carrying twodifferent amplified templates if the patches comprise differentsequences and the templates carry a number of universal ends. Nucleicacids will not spread between features in typical embodiments if theends of the templates in one feature are not complementary to the primersequences present in the adjacent feature. The use of said multiplenon-hybridizing template ends and features with multiple differentsequences is therefore advantageous in fabricating amplified arrayshaving a high density of features.

Beads used in certain embodiments of the invention may be of a diameter,width or length from 0.1 μm to 100 μm. Bead size can be selected to havereduced size, and hence get more features per unit area, whilstmaintaining sufficient signal (template copies per feature) in order toanalyze the features. The beads may be spherical, cylindrical, cubic orother three dimensional shape. The beads may be approximately 0.5-5microns in size, or may be reduced further in size, for example, ifdesired to approach the optical resolution of the instrument used forimaging the features. The beads may be at most 0.5 micron, 0.7 micron, 1micron, 2 microns, 5 microns, 10 microns or larger in diameter orcross-section. The average area occupied by a bead, and in turn theaverage size of a feature created from a bead in the methods set forthherein, can be, for example, at most about 1000 μm², 500 μm², 100 μm²,50 μm², 25 μm², 5 μm², 1 μm², 0.5 μm², 0.1 μm² or smaller. If desired,the variation between the area occupied by the beads may be tightlycontrolled, for example less than 25%, less than 10%, less than 5%, lessthan 2% or less than 1%.

Single stranded templates which are immobilized through their 5′ endregions to beads are available for hybridization through their 3′ endregions. If the templates have been amplified on the beads, then thesequence complementary to the 3′ end region of the template may beknown, for example, if the 3′ sequence is derived from one of theadapters attached to the target fragments during an amplification step.The 3′ end region sequence may also be known if it has been synthesizedas part of the bead-attached template. A surface to which beads are tobe immobilized can, therefore, carry a primer sequence complementary tothe 3′ end regions of the bead-attached templates.

A surface that is contacted with beads in a method according to certainembodiments of the invention can include more than one differentimmobilized primer species. For example, a surface can have 2, 3, 4, 5or more different primer species attached thereto. In some embodiments,one or more of the immobilized primers on a surface may carry acleavable moiety that can be used to invert a template strand on thesurface, for example, in a paired-end sequencing method as set forth infurther detail elsewhere herein or as described in co-pendingapplications U.S. Ser. No. 11/973,321, which stems from WO 08/041,002,the contents of which are incorporated herein by reference in theirentirety.

Beads can be immobilized to a surface by hybridization of abead-attached template to a surface-attached primer, and such a deviceis one embodiment of the invention. The invention may involve a deviceincluding an array of beads on a surface wherein the beads areimmobilized via hybridization to one or more oligonucleotide primers onthe surface. The surface may be a cross-linked polyacrylamide hydrogel,as described below, to which thiophosphate modified priming sequencesare attached, as shown below and in the examples section.

A method of the invention can include a step of primer extension inwhich a surface-attached primer is extended using a bead attachednucleic acid as a template. The term “primer extension” means making afirst nucleic acid molecule (i.e., the primer) longer by fabricating acopy of a template sequence of a nucleic acid that is hybridized to thefirst nucleic acid molecule. A single strand can be turned into a doublestrand by hybridizing a short sequence at one end, and extending theshort sequence. The extension can be performed using a polymerase andnucleoside triphosphates, or a ligase and a set of oligonucleotidecassettes of variable sequence. Primer extension can be carried out froma universal primer which hybridizes to a sequence common to multipledifferent nucleic acid templates, or can be carried out from a specificprimer that hybridizes to a sequence that is unique to a particulartemplate among different templates in a sample.

An exemplary method that uses primer extension to fabricate an array ofnucleic acids is shown in FIG. 2. In this embodiment beads havingattached templates can be selected by hybridization of the templates toprimers attached on the surface of another substrate. Beads that do nothave templates that complement the surface-attached primers will not beimmobilized and can be washed away. The primers on the surface can beextended using a nucleic acid polymerase and nucleotide triphosphates.Any nucleotide polymerase may be used including, for example, Taqpolymerase, or Phusion DNA polymerase which is a high fidelitypolymerase giving rise to fewer errors in the template strand.

The invention may thus in certain embodiments involve a compositionincluding an array of beads on a planar surface wherein the beads areimmobilized via hybridization to one or more oligonucleotide primers onthe planar surface, and a nucleic acid polymerase. The polymerase may bepresent in solution, and/or bound to the primer/template sequences onthe surface. The composition may further include one or more nucleosidetriphosphates. The composition may further include four nucleosidetriphosphates, which may be deoxynucleoside triphosphates.

In embodiments wherein one or more template nucleic acids aretransferred from a bead to a surface by primer extension, conditions forproducing no more than a single copy of each template molecule can beused. Alternatively, the templates can be copied by repeated cycles ofhybridization, extension and denaturation. This can provide theadvantage of creating localized sites of amplification on a surface. Forexample, if the surface carries primers that are complementary to thetemplate nucleic acids but does not carry a primer that is complementaryto the extended template, then the amplification can be localized at thebead-surface interface. More specifically, if the surface to which thebeads are immobilized carries a single type of primer (i.e., having thesame sequence, albeit in multiple copies), the extended copies can notbe amplified solely on that surface because a second primercomplementary to the extended copies is not present. In this example,since the DNA templates possess two distinct amplification sequences attheir two ends, they cannot spread out across the first surface bybridge amplification; however the templates and their complementarycopies can be further amplified via hybridization to complementaryprimers on the surface and on the beads such that a given templatemolecule is copied two or more times. In this way multiple copies of thetemplate are transferred to the surface within the interface area of thebead.

It has been found that double-stranded extension products formed at abead-surface interface can be denatured for further amplification,surprisingly, without resulting in migration of beads away from theinterface position, even under conditions where chemical denaturingagents are delivered to the bead-surface interface via fluid flow. Oncethe bead and surface are brought in close proximity, amplificationregions where the two surfaces overlap are created. Amplification cantake place where the two surfaces are in close proximity and thereforeboth primers are present and conditions can be used, for example, byappropriate primer choice as set forth above, such that copies arepresent only on the location of the surface that is in close proximityto a bead. In this way, the density of molecules per unit area can beincreased by performing one or more amplification cycles without theconsequent increase in diameter of the resulting feature on the surface.

Cycles of primer hybridization, extension and denaturation used totransfer a nucleic acid from a bead to a surface can generally becarried out as set forth herein in regard to amplification methods. Forexample denaturation can be achieved by thermocycling, exposure tochemical denaturants or action of strand separating enzymes. Any numberof desired cycles of primer hybridization, extension and denaturationcan be carried out including, for example, at least 2, 5, 10, 20, 25, 50or more.

Beads may be removed from contact with a surface after transfer ofcontent to the surface, or after the optional amplification of thecopied material. Returning to the example shown in FIG. 2, the beads canbe removed after templates on the beads have been copied by extension ofsurface-attached primers. In embodiments wherein the beads are onlyimmobilized via hybridization a protocol involving denaturation, such asheat or chemical treatment with formamide or hydroxide can be used toremove the beads from the surface to leave an array of extendedtemplates on the surface. The resulting array can be used in a nucleicacid analysis method such as those described elsewhere herein.

Templates that are present on a collection of beads may comprise a poolof nucleic acid fragments from a biological source, or may comprise apool of synthesized oligonucleotides. The biological source can be aeukaryotic unicellular or multicellular organism. Exemplary eukaryoticorganisms that are particularly useful include, without limitation, amammal such as a rodent, mouse, rat, rabbit, guinea pig, ungulate,horse, sheep, pig, goat, cow, cat, dog, primate, human or non-humanprimate; a plant such as Arabidopsis thaliana, corn, sorghum, oat,wheat, rice, canola, or soybean; an algae such as Chlamydomonasreinhardtii; a nematode such as Caenorhabditis elegans; an insect suchas Drosophila melanogaster, mosquito, fruit fly, honey bee or spider; afish such as zebrafish; a reptile; an amphibian such as a frog; adictyostelium discoideum; a fungi such as yeast; or a plasmodiumfalciparum. The biological source can also be an organism having asmaller genome such as a prokaryote, an archae, a virus, or a viroid.

References herein to a particular nucleic acid sequence may, dependingon the context, also refer to nucleic acid molecules which comprise thenucleic acid sequence. Sequencing of a target fragment means that a readof the order of bases is determined. The bases do not need to becontiguous, although this may be preferred, nor does every base on theentire fragment have to be sequenced. Sequencing using cycles ofligation where more than a single base varies between the ligationcassettes is within the scope of certain herein disclosed embodiments ofthe invention, and hence every cycle of sequencing does not have toresult in determination of a base for that cycle.

After content has been transferred from beads to a surface, theresulting features can be modified, for example, to facilitatesubsequent uses such as detection of the transferred content. Forexample, after bead-attached templates have been copied onto the surfaceof an array, the resultant nucleotide features can be further amplifiedon the surface. This can be advantageous if insufficient material hasbeen transferred from the beads. This may allow the use of smallerbeads, which carry less material. In such instances, the features afteramplification may be larger than the original beads. Amplification canbe carried at the features of an array using methods exemplified abovewith regard to amplifying nucleic acids on the surface of beads.However, amplification need not be carried out for nucleic acids infeatures of an array after the nucleic acids have been copied orotherwise transferred from beads.

Nucleic acids that are present in features that have been fabricatedusing a bead-based transfer method set forth herein can be used for anyof a variety of functions. Exemplary functions include, but are notlimited to, serving as a hybridization probe for a complementary nucleicacid target, serving as a primer for enzymatic extension, serving as aligation probe, serving as a template for an enzymatic extension orligation reaction, or serving as a substrate for a nucleic acid bindingmolecule such as a DNA binding protein or enzyme. It will be understoodthat the functions of a nucleic acid can change during different stepsof a method set forth herein. Accordingly, the nomenclature used torefer to a nucleic acid in one step may not necessarily limit thefunctions of the nucleic acid, or a copy thereof, with regard to othersteps. For example, a nucleic acid can be referred to as a templateduring a copying step and this template or its copy can subsequentlyserve as a primer or probe in another step. Exemplary methods thatexploit various functions of the nucleic acid features fabricated bybead-based transfer methods are set forth in greater detail below.

An array of nucleic acid features fabricated using the methods set forthherein may be further used in a number of biological assays including,for example, nucleic acid detection techniques such as genotyping, geneexpression analysis, methylation analysis or sequencing. Exemplarynucleic acid detection techniques that can be used are described below.It will be understood that a nucleic acid that is attached to an arrayis often referred to as a “probe” and a nucleic acid that is detected bythe probe is often referred to as a “target.” However, this is merely aconvention for ease of description and the terms can be usedinterchangeably with regard to the species of nucleic acid that isattached to an array feature and the nucleic acid that is detected at anarray feature. Accordingly, the description set forth herein with regardto arrays of templates or target nucleic acids are relevant to arrays ofprobes and vice versa.

Arrays that have been fabricated using beads are particularly useful formultiplex detection of nucleic acids due to the high density of probespresent on the arrays. The presence or quantity of particular sequencesin a sample can be determined, for example, based on hybridizationspecificity between the particular sequences and the probes present onan array. If desired, nucleic acid samples can be amplified prior tobeing contacted with an array, for example, using PCR methods, rollingcircle amplification methods, random prime amplification methods or thelike, in order to prepare copies of target nucleic acids in the sample.

A sample bearing target sequences can be contacted with an array ofprobes under conditions where target sequences hybridize tocomplementary probes and the hybrids can be detected to determine thepresence or quantity of target nucleic acids in the sample. Array baseddetection of hybrids can be carried out using methods known in the artsuch as those described in U.S. Pat. No. 6,355,431; US 2003/0211489, orUS 2005/0181394, each of which is incorporated herein by reference.Thus, multiplex detection methods can be used to detect the presence oftarget nucleic acids having specific sequences or to quantitate thenumber of such target nucleic acids in a sample, for example, as inexpression analysis.

In particular embodiments, target sequences can be detected in a samplebased on target-specific modification of probes or primers.Target-specific modifications include any modification of a primer orprobe that is indicative of a specific sequence present in a targetnucleic acid that hybridizes to the probe. Examples of target-specificmodifications include, for example, enzymatic modifications of primersand probes such as polymerase directed primer extension and ligasecatalyzed ligation of probe ends. Such methods are described in furtherdetail below in the context of genotyping reactions, but can be used forother purposes as well. The examples below are described in the contextof reactions involving a probe occurring at a feature on an array.However, it will be understood that a probe or primer can instead bemodified, for example in solution or on a separate solid phasesubstrate, and the modified probe can subsequently be detected at anarray feature. Thus, probe or primer modification need not occur on thearray where detection occurs.

Multiplex detection methods can involve detection at a resolution thatallows single nucleotide polymorphisms (SNPs) to be distinguished.Extension assays are useful for detection of alleles, mutations or othernucleic acid features at the resolution of SNPs, for example, todetermine genotypes. Extension assays are generally carried out bymodifying the 3′ end of a first nucleic acid when hybridized to a secondnucleic acid. In embodiments where a target nucleic acid is hybridizedto a surface attached probe, either the probe nucleic acid can beextended using the target nucleic acid as a template or the targetnucleic acid can be modified using the probe as a template.

Exemplary primer extension approaches that can be used include, forexample, allele-specific primer extension (ASPE) and single baseextension (SBE). Briefly, SBE utilizes an extension primer thathybridizes to a template nucleic acid at a location that is proximal oradjacent to a detection position, the detection position beingindicative of a single nucleotide polymorphism in the template. Apolymerase can be used to extend the 3′ end of the primer with anucleotide analogue labelled with a detection label. ASPE is anextension assay that utilizes extension primers that differ innucleotide composition at their 3′ end. Template-directed modificationof the 3′ portion of the primer, for example, by polymerase directedaddition of a labelled nucleotide yields a labelled extension product ifthe template includes the target sequence. The presence of the labellednucleotide in the extended primers from an SBE or ASPE reaction can bedetected at a particular location in an array and the added nucleotideidentified to determine the identity of a single nucleotidepolymorphism. Another primer extension method that can be used formultiplex detection on arrays is pyrophosphate detection, which isdescribed below in further detail in the context of sequencingapplications. SBE, ASPE and pyrophosphate based genotyping detection canbe carried out as described, for example, in US 2003/0108867; US2003/0215821; US 2003/0108900 or US 2005/0181394, each of which isincorporated herein by reference.

Multiplex detection with arrays can also include a ligation assay suchas oligonucleotide ligation assay (OLA) or ligation of pre-circle probesto form circular probes. Ligation assays involve the template-dependentligation of two probe ends. In embodiments where a target nucleic acidis hybridized to a surface attached probe, either the surface attachedprobe can be ligated to a second probe using the target nucleic acid asa template or an end of a target nucleic acid can be ligated to a secondprobe end using the surface attached probe as a template. In embodimentswhere the probe ends hybridize directly adjacent to each other, covalentlinkage can occur via a ligase. Alternatively, an extension-ligationassay can be used wherein hybridized probe ends are non-contiguous andone or more nucleotides are added along with one or more agents thatjoin the probe ends via the added nucleotides. A ligation assay orextension-ligation assay can be carried out with two separate probesthat become ligated together, or the assay can be carried out with asingle padlock probe that is ligated to become circular. A labelincorporated into a ligated product can be detected at an array featureto determine the presence of a single nucleotide polymorphism or othernucleic acid sequence characteristic. Exemplary conditions for ligationassays or extension-ligation assays are described, for example, in U.S.Pat. No. 6,355,431 and US 2003/0211489, each of which is incorporatedherein by reference.

In particular embodiments, modified probes such as ligation products caninclude priming sites configured to allow amplification of the modifiedprobe product using primers that hybridize to the priming sites, forexample, in a PCR reaction or rolling circle reaction. Universal primingsites are particularly useful. A modified probe such as a ligated probecan further include other features such as an adaptor sequence that isspecific for a probe on an array, restriction site for cleaving themodified probe or a tag sequence as described, for example, in U.S. Pat.No. 6,355,431 and US 2003/0211489, each of which is incorporated hereinby reference.

An array fabricated by the methods set forth herein can also be used todetect methylation of nucleic acids. In particular embodiments,methylated cytosines can be distinguished from non-methylated cytosinesbased on their differential reactivity with bisulfite in which case thelatter are converted to uracil and the former are protected fromconversion. Nucleic acids in a sample that has been treated withbisulfite can be detected using arrays as exemplified herein fordetecting single nucleotide polymorphisms or the nucleic acids can besequenced on arrays. Array detection is used to distinguish whether auracil is present at site expected to be cytosine, which is indicativeof unmethylated cytosine in the original sample, or whether a cytosineis present at such a site, which is indicative of a methylated cytosinein the original sample. Alternatively, methylation can be detected usingarrays to distinguish different fragments resulting from treating anucleic acid sample with methylation sensitive restrictionendonucleases. Useful methods for detecting methylation of nucleic acidswith arrays are described, for example, in US 2003/0170684, which isincorporated herein by reference.

The invention also encompasses methods of determining the identity ofone or more bases in the features derived by copying the templates onthe beads. The features on the surface may be identified using cycles ofhybridization and stripping of labelled oligonucleotides. The array ofknown sequences may be further used to analyze a biological sample atcertain positions in the sample.

The invention also encompasses, according to certain embodiments,methods of sequencing nucleic acid features that have been fabricatedusing methods set forth herein. For example, these and related inventionembodiments provide a method of nucleic acid sequencing comprisingamplifying a pool of nucleic acid templates on beads as described above,transferring the sequences to another surface by transferring or copyingthe templates and carrying out a nucleic acid sequencing reaction todetermine the sequence of the whole or a part of at least one nucleicacid feature on the surface.

Sequencing can be carried out using any suitable sequencing technique,wherein nucleotides are added successively to a free 3′ hydroxyl group,resulting in synthesis of a nucleic acid chain in the 5′ to 3′direction. The nature of the nucleotide added is preferably determinedafter each nucleotide addition. Sequencing techniques using sequencingby ligation, wherein not every contiguous base is sequenced, andtechniques such as massively parallel signature sequencing (MPSS) wherebases are removed from, rather than added to the strands on the surfaceare also useful, as are techniques using detection of pyrophosphaterelease (pyrosequencing). It will be understood that sequencing canproceed in a 3′ to 5′ direction, for example, in embodiments thatutilize sequencing by ligation.

The initiation point for a sequencing reaction may be provided byannealing of a sequencing primer to a target nucleic acid present at afeature of an array. In this connection, a known adapter region that ispresent on a target nucleic acid, for example, as a result of anamplification reaction described previously herein, can be used as apriming site for annealing of a sequencing primer.

In a particular embodiment, a nucleic acid sequencing reaction caninclude steps of hybridizing a sequencing primer to a single-strandedregion of a linearized amplification product, sequentially incorporatingone or more nucleotides into a nucleic acid strand complementary to theregion of amplified template strand to be sequenced, identifying thebase present in one or more of the incorporated nucleotide(s) andthereby determining the sequence of a region of the template strand.

One preferred sequencing method which can be used in accordance with theinvention relies on the use of modified nucleotides having removable 3′blocks, for example, as described in WO 04/018497 and U.S. Pat. No.7,057,026, the contents of which are incorporated herein by reference intheir entirety. Once the modified nucleotide has been incorporated intothe growing nucleic acid chain complementary to the region of thetemplate being sequenced there is no free 3′-OH group available todirect further sequence extension and therefore the polymerase can notadd further nucleotides. This allows convenient detection of singlenucleotide incorporation events. Once the identity of the baseincorporated into the growing chain has been determined, the 3′ blockmay be removed to allow addition of the next successive nucleotide. Byordering the products derived using these modified nucleotides, it ispossible to deduce the DNA sequence of the DNA template. Multiplereactions can be done in parallel on a single array, for example, ifeach of the modified nucleotides has a different label attached thereto,known to correspond to the particular base, thereby facilitatingdiscrimination between the bases added during each incorporation step.If desired, a separate reaction may be carried out for each of themodified nucleotides.

Modified nucleotides used in an amplification or sequencing reaction maycarry a label to facilitate their detection. A fluorescent label, forexample, may be used for detection of modified nucleotides. Eachnucleotide type may thus carry a different fluorescent label, forexample, as described in U.S. Provisional Application No. 60/801,270(Novel dyes and the use of their labelled conjugates), published as WO07/135,368, the contents of which are incorporated herein by referencein their entirety. The detectable label need not, however, be afluorescent label. Any label can be used which allows the detection ofan incorporated nucleotide. Similarly, fluorescent labels or otherlabels can be used to detect any of a variety of analytes on an arrayfabricated using a bead-based transfer method set forth herein.

One method for detecting fluorescently labelled nucleotides comprisesusing laser light of a wavelength specific for the labelled nucleotides,or the use of other suitable sources of illumination. The fluorescencefrom the label on the nucleotide may be detected by a CCD camera orother suitable detection means. Suitable instrumentation for recordingimages of clustered arrays is described in U.S. Provisional ApplicationNo. 60/788,248 (Systems and devices for sequence by synthesis analysis),published as WO 07/123,744, which entered the U.S. national phase asU.S. patent application Ser. No. 12/295,337, each of which areincorporated herein by reference in their entirety. Detectors that arecapable of obtaining an image of an array surface such as thoseconfigured to scan an array surface. Such detectors can be configured totake a static image of an array surface, scan a point across an arraysurface or scan a line across an array surface. Exemplary scanningdevices that can be used are described, for example, in U.S. Pat. No.7,329,860, which is incorporated herein by reference. A detector can beconfigured to obtain an image of an array at high resolution, forexample, in the low micron to submicron range. In particularembodiments, an image can be obtained at a Rayleigh resolution between0.2 and 10 micrometers.

The invention is not intended to be limited to use of the sequencingmethods outlined above, as a variety of sequencing methodologies whichutilize successive incorporation of nucleotides into a nucleic acidchain or removal of nucleotides from a nucleic acid chain can be used.Suitable alternative techniques include, for example, PyrosequencingFISSEQ (fluorescent in situ sequencing), MPSS and sequencing byligation-based methods, for example as described in U.S. Pat. No.6,306,597. Sequencing by hybridization methods can also be used.

A nucleic acid may be analyzed to obtain a first and then a secondsequencing read from opposite ends of the nucleic acid. Methodology forsequencing both ends of nucleic acids at array features (also referredto as “clusters”) are described in co-pending applications WO07/010,252, which entered the US national phase as U.S. patentapplication Ser. No. 11/989,172 and WO 08/041,002 and U.S. 2009/0088327,the contents of which are incorporated by reference herein in theirentirety. These methods utilize a step of copying a first templatestrand by hybridizing the 3′ end to an immobilized primer followed byextending the resulting bridged structure to generate a second templatestrand. This copying step can be carried out after the template has beensequenced from a first end. Then the first strand can be cleaved fromthe surface and the remaining second template strand can be sequencedfrom the other end. In order to practice this embodiment of theinvention, two or more immobilized primers are utilized, at least one ofwhich is configured to be cleavable in order to release the firsttemplate strand.

One or more of the sequencing reads may be carried out on the layer ofbeads while the beads are in contact with the surface. In particularembodiments, the nucleic acid material on the beads can be sequenced insitu either before or after the nucleic acid material is transferred tothe surface. Alternatively, the nucleic acid material on the beads canbe sequenced in situ and the material need not be transferred to thesurface at all.

Taking as an example an embodiment where nucleic acid material issequenced in situ either before the material is transferred to thesurface, the beads may carry templates which are used to capture thebeads by hybridization, and the templates copied to the surface byextension. The beads may carry additional single stranded templatecopies which did not hybridize to the support bound primers, and thusremain single stranded and available for sequencing. A first sequencingread may therefore be carried out on the beads using the beadimmobilized templates and a non-immobilized primer hybridized to thetemplates. The beads may then be removed from the support, for exampleby denaturing the duplexes produced by the initial extension, to leavean array of features which are single-stranded copies of the templateson the beads. The features on the array may then be sequenced togenerate a second read. The first read on the bead, and the second readon the array may originate from opposite ends of the original templates,and thus provide paired read information from both ends of thetemplates.

With a significant proportion of human genomic DNA being composed ofrepetitive sequence, it can be beneficial to reduce the complexity ofthe sample in order to reduce the amount of sequencing required toidentify particular sequences of interest. Furthermore, with priorgenetic information, it is possible to correlate a phenotype, such as apredisposition to a disease, with the genetic variation of one or moreregions of the genome. In some applications of genetics, what is desiredis the application and advantages of high throughput sequencing methodsspecifically to particular regions of interest among many individuals.In addition, in certain circumstances, it is desirable to generate a‘genome-wide’ analysis of a particular subset of genomic features, suchas exons, chromosomes etc., in one or more individuals. For examplegenome-wide analysis of exons can be useful to correlate geneticdiversity in the protein-coding regions across many individuals. Suchanalyses can be carried out using certain of the invention embodimentsdescribed herein in a number of ways, several of which are describedbelow.

For example, a pool of different bead-bound oligonucleotides can beprepared in which a single oligonucleotide is attached to eachindividual bead. The oligonucleotides can be in the form of RNA or DNA.The bead can be used as bait in order to selectively hybridize tocomplementary sequences from solution. The hybridized material can becopied on the beads using, for example, methods described hereinpreviously. The beads can then be contacted with a surface such that theends of the copied material hybridize to surface-attached primers andthen copies of the bead bound material can be made by extension of theprimers.

Alternatively two or more primer sequences can be attached to each bead,and the beads used to selectively bind to flanking regions of targetsequences such that the primers can be used to amplify the region of atarget nucleic acid that resides between the flanking primer bindingsites. This negates the need for emulsion amplification, as theamplification on each bead is sequence selective, and avoids diffusionof templates between beads since the priming sequences are differentbetween different beads. If the primers also have a universal region atthe 5′ ends, then the ends of the different amplified sequences will bethe same, and thus amenable to capture on a surface carrying a singleprimer species.

Templates that are present on beads may be amplified from a pool ofchemically synthesized oligonucleotides. Thus the beads may begin with asingle primer species attached to all the beads, but if a collection ofoligonucleotide probes is prepared which have universal endscomplementary to the primers on the beads and variable central portions,then individual oligonucleotide molecules can be amplified on beads inthe same way as individual strands of nucleic acid strands from abiological source. The oligonucleotide amplicons can be captured ontothe surface and used to form a high density array of nucleic acidfeatures. These features can have their sequences decoded by cycles ofhybridization using labelled nucleic probes. Once the features aredetermined, they can be used for sequencing or genotyping as describedabove.

An array fabricated by a method set forth herein can be used to capturetarget sequences by hybridization. The captured target sequences canthen be amplified using the sequences copied from the beads asamplification primers. As the transfer from the beads gives nucleic acidsequences at discrete locations, if the captured sequences areamplified, they are unable to spread outside the area of the feature asthe spaces between the features do not carry any amplification primers.A resulting advantage is the regular spacing of nucleic acid features ona surface that may be suitable for subsequent amplification. The nucleicacid features produced may comprise amplification primers, which may beof a single sequence per feature, or two or more sequences per feature.The sequences may be universal sequences common to all features, or maybe different between different features. A further embodimentcontemplates the use of the amplification primers on the surface for theamplification of template molecules. The amplified template moleculesmay comprise substantially all the primer sequences in the feature, asthe amplification reaction can be pushed through a large number ofcycles as the features are unable to spread into areas where there areno primers. Thus the amplified features may be of similar size andintensity to each other, as each feature will be grown from a similarnumber of primers which are fabricated on the surface using methodsdescribed herein.

Methods for bead-based transfer of content to a surface and relatedcompositions are exemplified herein with regard to transfer of nucleicacid content for ease of description. However, it will be understoodthat other content can be used in the methods and compositionsincluding, without limitation, proteins such as antibodies, enzymes orreceptors; peptides; saccharides; synthetic molecules such as candidateenzyme cofactors, enzyme inhibitors, enzyme activators, enzymesubstrates; drug candidates or the like. Useful enzymes include forexample, kinases, phosphatases, proteases, nucleases, polymerases,lipases, reductases and others known in the art.

The invention is herein further described by reference to the followingnon-limiting examples.

EXAMPLES

The following are examples of general techniques which may be applied incarrying out the method of the invention.

The following experiment, drawn as a cartoon in FIG. 3, shows theefficient transfer of material from a bead to a surface, removal of thebeads, and cycles of sequencing to show that the transferred templateson the surface retain their sequence integrity and do not overlap.

1. Two PCR products (CT417 (SEQ ID NO: 5)) and CT418 (SEQ ID NO: 6))were prepared as shown in FIG. 4.

The two products carried universal ends, and were prepared using asingle pair of primers; namely:

Primer A (P7-OH): (SEQ ID NO: 1) 5′-CAAGCAGAAGACGGCATACGA Primer B (5′biotinylated-P5-SBS3): (SEQ ID NO: 2)5′-biotin-AATGATACGGCGACCACCGAGATCTACACTCTTTCCCTA CACGACGCTCTTCCGATCxTWhere x=phosphorothioate.

The template strands were amplified using a PCR reaction as follows:

Reagent solution:

1 μl ( 1/50) dilution of 10 nM stock DNA (CT417 or CT418)

Primer A, final conc 0.5 μM

Primer B, final conc 0.5 μM

25 μl Phusion HF 2× mastermix

H₂O to final volume 50 μl

Cycled for 25 cycles of 94 C for 10 s; 65 C for 30 s; 72 C for 30 s;Final extension of 72 C for 5 min

2. The templates were bound to streptavidin beads as follows: (note: allmanipulations of beads were done using a magnet)

-   -   a. 20 μl of MyOne C1 beads (Dynal), were prewashed twice with        1×B/W buffer (Binding and Wash buffer, manufacturer's        recommendation)    -   b. Resuspended the beads in 25 μl of 2×B/W buffer and added 25        μl of biotinylated PCR product    -   c. Incubated room temp for ˜20 mins    -   d. Pulled down beads, discarded supernatant and wash with 200 μl        1×B/W    -   e. Washed again with 200 μl 1×B/W    -   f. Washed with 200 μl Wash buffer (Illumina: 0.3×SSC/0.1% Tween)    -   g. Made DNA bound to beads single-stranded by washing with 200        μl 0.1 N NaOH    -   h. Pulled down, discarded denatured strand, and neutralized by        washing with 200 μl Hybridization buffer (Illumina: 5×SSC/0.1%        Tween)    -   i. Washed again with 200 μl Hybridization buffer    -   j. Transferred beads in Hyb buffer to fresh tubes    -   k. Washed with 200 μl Wash buffer    -   l. Resuspended beads in 20 μl Wash buffer

3. The beads from step 2 were flushed into the flowcell and printed asfollows:

-   -   a. Prepared a standard paired end flowcell (Illumina, San Diego,        Calif.) (preparation of Illumina flow cells is described below,        and in full in co-pending applications WO2008/002502 and U.S.        application Ser. No. 11/973,321 (corresponding to        WO2008/041002), the contents of which are incorporated herein by        reference in their entirety.    -   b. Flushed through 120 μl of Hyb buffer at 60 μl/min, 20 C    -   c. Increased temp to 96 C, pumped through a further 75 μl of Hyb        buffer at 15 μl/min    -   d. Cooled to 40 C    -   e. Flushed through 75 μl of Wash buffer at 15 μl/min at 40 C    -   f. Pumped a small air bubble into the lines (5 μl)    -   g. Flushed in beads prepared in step 2 at 15 μl/min (10 μl beads        per channel)    -   h. Stopped pumping when the beads have filled the channels of        the flowcell    -   i. Incubated at 40 C for 10 mins    -   j. Flushed out unbound beads with Wash buffer at 40 C, 15 μl/min        for 75 μl    -   k. Pumped in Taq extension mix (2 M betaine, 20 mM Tris, 10 mM        Ammonium Sulfate, 2 mM Magnesium sulfate, 0.1% Triton, 1.3%        DMSO, pH 8.8 plus 200 μM dNTP's and 25 units/mL of Taq        polymerase (NEB Product ref M0273L)) at 15 μl/min for 75 μl    -   l. Heated to 74 C for 90 s to copy strands from beads onto        surface oligos    -   m. Cooled to 60 C    -   n. Removed beads from flowcell by flushing in formamide at 60 C,        15 μl/min for 75 μl, then 0.1 N NaOH at 20 C, 60 μl/min for 75        μl

The features on the surface can be further copied using the followingoptional steps:

-   -   o. Pump in Taq extension mix (same recipe as above), 60 μl/min        for 75 μl    -   p. Heat to 74 C for 90 s to create the 2^(nd) strand of each        “bridge”    -   q. SyBr Green stain and scan printed material

Before the final removal step (step 2(n) above), beads were observed onthe surface using a white light imaging system. Pictures of the beads inthe flow cell are shown in FIG. 5. Beads without templates did not stickto the surface, showing that the capture was specifically via templatehybridization (see FIG. 5F).

FIG. 6 shows images of a number of channels of the flow cell which werehybridized with a primer complementary to the ends of the template andextended using a cycle of sequencing. Where the beads carried a singletemplate, the appropriate nucleotide was incorporated. Mixtures of beadscarrying monotemplates gave discrete regions of sequence at theappropriate ratio depending on the number of each bead types. Beads thatcarried a mixture of the two templates gave a mixture of both bases.FIG. 7 shows a high resolution image of features on a surface derivedfrom a one micron bead. The features were tightly packed on the surface,and show discrete areas of non-overlapping signal, with each spotshowing similar size and intensity.

The experiments detailed below show the conditions used forhelicase-dependent amplification on a pool of beads:

3. Streptavidin beads were grafted with biotinylated primers A and B asfollows (note: all manipulations of beads were done using a magnet):

-   -   a. 80 μl of MyOne Cl beads (Dynal) were prewashed twice with        1×B/W buffer (Binding and Wash buffer, manufacturer's        recommendation)    -   b. Resuspended the beads in 100 μl of 2×B/W buffer+50 μl of        primer A and 50 μl of primer B    -   c. Incubated room temp for ˜20 mins    -   d. Pulled down beads, discarded supernatant and washed with 200        μl 1×B/W    -   e. Washed again with 200 μl 1×B/W

4. The beads from step 3 were then hybridized to templates from step 1and the attached primers extended as follows:

-   -   a. Gel-purified PCR products from step 1 above (eluted in buffer        EB (QIAGEN)) were taken, Tween20 added to 0.1% and denatured by        heating to 98 C for 5 mins and snap chilling on ice    -   b. Added 20×SSC to give a final concentration of 5×SSC in the        sample    -   c. Incubated the denatured DNA at 45° C. and add 10 μl of        grafted beads from step 3    -   d. Continued incubation at 45° C. for 15 mins    -   e. Washed beads three times in 200 μl pre-warmed Wash buffer        (0.3×SSC/0.1% Tween) at 45° C.    -   f. Washed beads in 200 μl extension pre-mix (2 M betaine, 20 mM        Tris, 10 mM Ammonium Sulfate, 2 mM Magnesium sulfate, 0.1%        Triton, 1.3% DMSO, pH 8.8)    -   g. Resuspended the beads in 100 μl Taq extension mix ((2 M        betaine, 20 mM Tris, 10 mM Ammonium Sulfate, 2 mM Magnesium        sulfate, 0.1% Triton, 1.3% DMSO, pH 8.8 plus 200 μM dNTP's and        25 units/mL of Taq polymerase (NEB Product ref M0273L))    -   h. Transferred to 0.2 ml tubes and heat to 74° C. for 180 s    -   i. Cooled to room temp and washed beads three times with 200 μl        0.1N NaOH to remove seeding strands    -   j. Washed beads with 200 μl Hybridization buffer (5×SSC/0.1%        Tween)    -   k. Washed beads with 200 μl Wash buffer    -   l. Washed beads with 200 μl extension pre-mix    -   m. Washed beads with 400 μl extension pre-mix    -   n. Resuspended in 50 μl tHDA mix (BioHelix)—1.75 μl 100 mM MgSO4        (3.5 mM final), 10 μl 5M betaine (1M final), 13.25 μl H₂O, 25 μl        2×tHDA mix    -   o. Incubated at 64° C. for 3 hrs    -   p. Washed beads with 200 μl Wash buffer

Illumina standard paired end flow cells were prepared as described inoutline below, and as described in full in co-pending applications WO08/002,502 and U.S. application Ser. No. 11/973,321 (corresponding to WO08/041,002), the contents of which are incorporated herein by referencein their entirety:

Glass 8-channel flow cells (Silex Microsystems, Sweden) were thoroughlywashed and then coated for 90 min at 20° C. with 2% acrylamidecontaining ˜3.9 mg/ml N-(5-bromoacetamidylpentyl) acrylamide, 0.85 mg/mltetramethylethylenediamine (TEMED) and 0.48 mg/ml potassium persulfate(K₂S₂O₈). Flow cell channels were rinsed thoroughly before further use.The coated surface was then functionalized by reaction for 1 hour at 50°C. with a mixture containing 0.5 μM each of two primers (primers C and Dshown below) in 10 mM potassium phosphate buffer pH 7. Grafted flowcells were stored in storage buffer (5×SSC) until required.

primer ‘C’: (SEQ ID NO: 3) 5′PS TTTTTTTTTTAATGATACGGCGACCACCGAGAUCTACAC-3′ (“PS” =5′-thiophosphate; U = 2-deoxyuridine) primer ‘D’: (SEQ ID NO: 4) 5′PS TTTTTTTTTTCAAGCAGAAGACGGCATACGAGoxoAT-3′, (“PS” =5′-thiophosphate; “Goxo” = 8-oxoguanine)

While the foregoing invention has been described in some detail forpurposes of clarity and understanding, it will be clear to one skilledin the art from a reading of this disclosure that various changes inform and detail can be made without departing from the true scope of theinvention. For example, all the techniques and apparatus described abovemay be used in various combinations. All publications, patents, patentapplications, or other documents cited in this application areincorporated by reference in their entirety for all purposes to the sameextent as if each individual publication, patent, patent application, orother document were individually indicated to be incorporated byreference for all purposes.

As used herein, the term “each” when used in reference to a collectionof items is intended to identify one or more individual items in thecollection but does not necessarily refer to every item in thecollection unless the context clearly dictates otherwise.

1. A method of creating an array of features, comprising (a) providing aplurality of beads, wherein each bead in the plurality of beadscomprises nucleic acid content; (b) contacting the plurality of beadswith a surface to produce a layer of beads on the surface; (c)transferring the nucleic acid content from the beads to the surface tocreate an array of spatially discrete features on the surface, whereineach spatially discrete feature comprises nucleic acid content from abead in the plurality of beads; and (d) performing an amplificationreaction using the nucleic acid content at each of the discrete featureson the surface, thereby producing a homogenous population of nucleicacids at each feature.
 2. The method of claim 1, wherein thetransferring comprises physically transferring nucleic acids from thebeads to each of the spatially discrete features on the surface.
 3. Themethod of claim 1, wherein the transferring comprises covalentlyattaching nucleic acids from the beads to each of the spatially discretefeatures on the surface.
 4. The method of claim 1, wherein thetransferring comprises non-covalently attaching nucleic acids from thebeads to each of the spatially discrete features on the surface.
 5. Themethod of claim 1, wherein a single nucleic acid molecule is transferredfrom each of the beads to each of the spatially discrete features. 6.The method of claim 1, wherein several copies of a single nucleic acidspecies are transferred from each of the beads to each of the spatiallydiscrete features.
 7. The method of claim 1, wherein the transferringcomprises replicating a nucleic acid from each of the beads to form acopy of the nucleic acid at each of the spatially discrete features. 8.The method of claim 1, wherein the transferring comprises physicallytransferring a nucleic acid molecule from each of the beads to attachthe nucleic acid molecule to each of the spatially discrete features. 9.The method of claim 1, wherein the nucleic acids that are transferredfrom the beads to the surface are single stranded.
 10. The method ofclaim 1, wherein the nucleic acids that are transferred from the beadsto the surface are double stranded.
 11. The method of claim 1, whereinthe array of spatially discrete features comprises an averagecenter-to-center spacing that is equivalent to the diameter of thebeads.
 12. The method of claim 1, wherein the array of spatiallydiscrete features comprises an average center-to-center spacing that isless than 100 μm.
 13. The method of claim 1, wherein the surface is flator planar.
 14. The method of claim 1, wherein the surface compriseswells, depressions, pillars, ridges or channels.
 15. The method of claim1, further comprising removing the beads from the surface after thetransferring of the nucleic acid content from the beads to the surface.16. The method of claim 15, further comprising obtaining an image of thesurface after removing the beads from the surface.
 17. The method ofclaim 1, wherein the nucleic acid content comprises nucleic acid primersthat are transferred from the beads to the surface and used as primersin the amplification reaction.
 18. The method of claim 17, whereindifferent beads in the plurality of beads comprise primers having thesame sequence.
 19. The method of claim 17, wherein different beads inthe plurality of beads comprise primers having different sequences. 20.The method of claim 17, wherein each of the beads in the plurality ofbeads comprises two amplification primers each having a differentsequence.
 21. The method of claim 1, wherein the nucleic acid contentcomprises nucleic acid templates that are transferred from the beads tothe surface and used as templates that are copied in the amplificationreaction.
 22. The method of claim 1, wherein nucleic acid content oneach bead comprise a nucleic acid molecule having (i) a universalsequence that is the same as the universal sequence on other beads inthe plurality of beads and (ii) a region of sequence that is differentfrom a region of sequence on the other beads in the plurality of beads.23. The method of claim 1, wherein the amplification reaction isperformed using a universal primer that is complementary to a universalsequence that is the same as the universal sequence on other beads inthe plurality of beads.
 24. The method of claim 1, wherein theamplification reaction uses primers attached to the surface.
 25. Themethod of claim 1, further comprising sequencing nucleic acids in thehomogenous population of nucleic acids at each feature.
 26. The methodof claim 1, further comprising hybridizing nucleic acid probes tonucleic acids in the homogenous population of nucleic acids at eachfeature.
 27. The method of claim 26, further comprising detecting alabel at features on the surface, wherein the label is attached to theprobes or attached to the nucleic acids in the homogeneous population ofnucleic acids.